# std::ranges::sized_range, std::ranges::disable_sized_range

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< cpp‎ | ranges

Defined in header `<ranges>` |  |
template< class T > concept sized_range = ranges::range<T> && requires(T& t) { ranges::size(t); }; | (1) | (since C++20)
template< class > constexpr bool disable_sized_range = false; | (2) | (since C++20)

1) The `sized_range` concept specifies the requirements of a `range` type that knows its size in constant time with the `size` function.
2) The `disable_sized_range` exists to allow use of range types that provide a `size` function (either as a member or as a non-member) but do not in fact model `sized_range`. Users may specialize `disable_sized_range` for cv-unqualified program-defined types. Such specializations shall be usable in constant expressions and have type const bool.

### Semantic requirements

1) Given an lvalue `t` of type std::remove_reference_t<T>, `T` models `sized_range` only if

- ranges::size(t)

  - has amortized constant-time complexity,
  - does not alter the value of `t` in a manner observable to equality-preserving expressions, and
  - is equal to ranges::distance(ranges::begin(t), ranges::end(t)), and

- if ranges::iterator_t<T> models `forward_iterator`, ranges::size(t) is well-defined regardless of the evaluation of ranges::begin(t) (in other words, a single-pass sized range may support a call to size only before the first call to begin, but a forward range must support size at all times).

### Notes

`disable_sized_range` cannot be used to opt-out a range whose iterator and sentinel satisfy `sized_sentinel_for`; std::disable_sized_sentinel_for must be used instead.

`disable_sized_range` cannot be specialized for array types or reference types.

### Example

Run this code

    #include <forward_list>
    #include <list>
    #include <ranges>

    static_assert
    (
        std::ranges::sized_range<std::list<int>> and
        not std::ranges::sized_range<std::forward_list<int>>
    );

    int main() {}

### See also

ranges::random_access_range (C++20) | specifies a range whose iterator type satisfies `random_access_iterator` (concept)
ranges::contiguous_range (C++20) | specifies a range whose iterator type satisfies `contiguous_iterator` (concept)

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